The addition of dextran plus sucrose to cell-free culture supernatants of Streptococcus mutans resulted in the formation of a semi-solid gel closely resembling 2% agar. This phenomenon is dependent upon an extracellular factor(s) which is referred to as "gel factor" (GF). The GF is produced by cells grown in complex medium with glucose, maltose or fructose as the primary energy source. GF appears to be protein in nature; it is heat labile and is inactivated by trypsin. The GF produced by S. mutans strains belonging to Bratthall's group "d" but not by strains belonging to group "a", "b" or "c". The potential role of this factor and its product in colonization and cell adhesion is being assessed.